Implantable medical devices are used to provide therapy to patients suffering from a variety of conditions. Examples of implantable medical devices are pacemakers and cardioverter-defibrillators (ICDs), which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers, when necessary. For example, pacemakers are designed to sense arrhythmias, i.e., disturbances in heart rhythm, and in turn, provide appropriate electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to correct the arrhythmias and restore the proper heart rhythm. The types of arrhythmias that may be detected and corrected by pacemakers include bradycardias, which are unusually slow heart rates, and certain tachycardias, which are unusually fast heart rates.
Cardioverter-defibrillators (ICDs) also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct the abnormal heart rate. In contrast to pacemakers, however, an ICD can also provide pulses that are much stronger and less frequent. This is because ICDs are generally designed to correct fibrillation, which is a rapid, unsynchronized quivering of one or more heart chambers, and severe tachycardias, where the heartbeats are very fast but coordinated. To correct such arrhythmias, ICDs deliver low, moderate, or high-energy shocks to the heart.
The electrical energy for the shocks generated by ICDs is delivered to the heart via electrical stimulation electrodes. One or more capacitors within the ICD are capable of rapidly delivering that energy to the patient's heart through leads that electrically connect the capacitors to the electrodes. In order to provide timely therapy to the patient after the detection of ventricular fibrillation, for example, it is necessary to charge the capacitors with the required amount of energy as quickly as possible. Thus, a battery in an ICD generally has a high rate capability to provide the necessary current to charge the capacitors.
In some designs, ICDs use large surface area electrodes either to be placed endocardially, epicardially or subcutaneously. One well-known approach of providing a large surface area electrode is to employ an elongated exposed coil of biocompatible metal. As is known, such elongated coils can be used with a wide variety of leads. For example, with an epicardial lead, an elongated coil serving as the electrode can be mounted around the exterior of an insulative lead body. In this context, it has been desirable to stabilize the electrode coil with respect to the lead body, both to provide mechanical integrity and to prevent fibrous growth around the individual coils of the electrode coil. In some designs, this has been accomplished by sliding the electrode coil over the lead body and backfilling spaces between the coil and the lead body with a plastic material. The exterior surface of the electrode coil is then machined to provide a smooth surface. Alternatively, the backfilling material may be removed by means of well-known plasma etching methods. Generally, the process can be varied as desired in order to provide the warranted amount of exposed surface area for the coil wire. For example, the removal process may be provided so that the backfilling material only extends radially outward between the turns of the coil from about one-third to one-half the diameter of the coil wire.
Alternative methods of making similar defibrillation lead structures without the necessity of using a backfilling can employ materials such as polyurethane to stabilize the electrode coil and to fill between the turns of the coil. In certain methods, a plastic tube can be stretched so that it displays an inner and outer diameter less than the inner and outer diameter of the tube in a relaxed state. An electrode coil having a inner diameter less than the outer diameter of the tube in its relaxed state is then slide over the stretched tube, after which the tube is released, allowing it to return to its previous length. However, after such release, the tube remains in a partially compressed state because of its contact with the electrode coil throughout the coil's length. Thereafter, a mandrel having an outer diameter greater than the inner diameter of the tubing in its compressed state is passed into the tubing, to further compress the tubing between the mandrel and the conductor coil. The assembly is thereafter heated, allowing the tubing to flow into spaces between the electrode coil.
While the methods of providing a plastic or polyurethane material between the electrode coil and lead body described above have been generally used, each has shortcomings. One particular shortcoming, with respect to both methods, is that there is variability in the pressure applied to the plastic or polyurethane material when situated between the individual coils of the electrode coil. As such, an uneven structure is often produced in which the plastic or polyurethane material does not flow outward to a consistent dimension between the individual coils. The present invention is directed to overcoming, or at least reducing the effects of, this shortcoming as well as others.